This disclosure relates to a current sensing system calibration method and apparatus, and more particularly, to a current sensing system calibration method and apparatus for a current sensing system that is used for controlling a motor.
Current sensors are known in the art for monitoring current signals. FIG. 1 depicts a conventional current sensing system generally denoted by the numeral 10. An input current 12 flows through a current sensing resistance 14, such as a shunt resistance of a motor wiring. A first end 16 of a pair of input resistances 18 is connected to both ends of the current sensing resistance 14. Input resistances 18 may be used to control the input impedance of the current sensing system 10. Each of the input resistances 18 has a second end 20 connected to a filter 22. The current 12 through current sensing resistance 14 establishes a voltage 24 at filter 22. The filter 22 has a pair of output leads 26 connected to an amplifier 28. The amplifier 28 generates an output 30 indicative of the current 12.
The current sensing resistance 14 needs to have a reference value so that the voltage 24 input at filter 22 accurately reflects the current 12. Variations in the current sensing resistance 14 from a reference value will cause variations in the voltage 24 thus producing inaccurate current measurements. Several techniques have been proposed to calibrate the current sensing system 10 to accommodate the value of the current sensing resistance 14.
One calibration technique is to change gain resistors in amplifier 28. Changing gain resistance in the amplifier, however, may not be practical in certain situations. For example, the gain resistor may be internal to an integrated circuit (IC). The addition of further gain resistors external to the IC would affect gain accuracy and common mode rejection due to variability of the on-chip resistances over build and temperature.
Another technique for calibration of the current sensing system 10 involves changing or trimming the current sensing resistance 14 to alter its resistive value. Trimming can be difficult because of the high power involved (80 Amps, 2 milliohms). For example, in a situation wherein a blade shunt is used as the current sensing resistance 14, trimming would most likely damage the current sensing resistance. Resistors such as thick film shunt resistance are expensive and tend to be unsuitable for trimming.
Yet another technique for calibrating the current sensing system 10 is to use software to adjust the signal generated by amplifier 28. One drawback to the software approach, among others, is that it hurts resolution.
Disclosed is a current sensing system comprising a current sensing resistance through which a current to be measured flows. The system further has a calibration resistance coupled to the current sensing resistance and a pair of input resistances each having a first end coupled to one of the ends of the current sensing resistance and a second end coupled to one of the ends of the calibration resistance. A voltage is created across the calibration resistance and the calibration resistance is set at a predetermined level to compensate for any deviation of the current sensing resistance from a reference value.
Also disclosed is a motor driving system that uses the current sensing system of the invention to measure a DC bus current.
Also disclosed is a method for calibrating the current sensing system.